LED system with driver voltage clamping

ABSTRACT

One example includes a light-emitting diode (LED) system. The LED system includes an LED array comprising a plurality of LEDs that are each activated via an LED current provided therethrough to provide illumination. The system also includes an LED controller configured to sequentially activate the plurality of LEDs via at least one LED driver system configured to selectively provide the LED current through each sequential one of the plurality of LEDs in an activated state in response to an activation signal. The at least one LED driver system includes a voltage clamp configured to maintain a substantial constant amplitude difference of a driver voltage associated with the LED current from the activated state to a deactivated state.

Under 35 U.S.C. §§ 120, 121 this continuation application claims benefits of and priority to U.S. patent application Ser. No. 15/955,511 (TI-73950.1), filed on Apr. 17, 2018, which is a divisional application of U.S. Nonprovisional application Ser. No. 14/569,094, filed on Feb. 28, 2014 (now U.S. Pat. No. 9,949,327), which claims the benefit of PCT Application No. PCT/CN2014/072691, filed Feb. 28, 2014. The above referenced applications are hereby incorporated herein by reference in their entirety.

TECHNICAL FIELD

This disclosure relates generally to electronic circuit systems, and more specifically to an LED system with driver voltage clamping.

BACKGROUND

The use of light-emitting diode (LED) strings instead of fluorescent bulbs for use in illumination of a backlight for a display, such as a television, a monitor for a laptop computer, or an LED wall, is increasing drastically based on consumer demands for better picture quality. In addition, typical LED light efficacy can be much better than conventional lighting systems for such displays, thus consuming significantly less power. In addition, among other advantages, LED systems can be smaller and more environmentally friendly, and can have a faster response with less electro-magnetic interference (EMI) emissions. A number of LED regulation techniques exist for typical LED systems. A typical LED system that can be implemented for a display can utilize different colored LEDs, such as red, green, and blue. However, delays between activations of LEDs in a given sequence, such as in scanning by rows, can result in a lack of uniformity in intensity across the display, especially in low grayscale conditions.

SUMMARY

One example includes a light-emitting diode (LED) system. The LED system includes an LED array comprising a plurality of LEDs that are each activated via an LED current provided therethrough to provide illumination. The system also includes an LED controller configured to sequentially activate the plurality of LEDs via at least one LED driver system configured to selectively provide the LED current through each sequential one of the plurality of LEDs in an activated state in response to an activation signal. The at least one LED driver system includes a voltage clamp configured to maintain a substantial constant amplitude difference of a driver voltage associated with the LED current from the activated state to a deactivated state.

Another example includes a method for providing substantially uniform illumination in an LED system. The method includes sequentially activating a plurality of rows of LEDs arranged in an array. The method also includes activating a switch in response to an activation signal to conduct an LED current through an LED in a respective activated one of the plurality of rows of LEDs to activate the LED. The switch can be coupled to the LED via an intermediate node having a driver voltage. The method also includes deactivating the switch via a deactivation signal to deactivate the LED. The method further includes conducting a clamping current from the intermediate node to set a maximum amplitude of the driver voltage while the switch is deactivated.

Another embodiment includes an LED system. The system includes an LED array comprising a plurality of LEDs arranged in a plurality of rows and a plurality of columns that are each activated via an LED current provided therethrough to provide illumination. The system also includes an LED controller. The LED controller includes an activation controller configured to sequentially activate each of the plurality of rows and to provide activation signals in response to a digital input. The LED controller also includes a plurality of LED driver systems that are each associated with a respective one of the plurality of columns. Each of the plurality of LED driver systems can be configured to activate one of the plurality of LEDs in a respective activated one of the plurality of rows in response to a respective one of the activation signals. Each of plurality of LED driver systems can include a voltage clamp configured to maintain a substantial constant amplitude difference of a driver voltage associated with the LED current between activation of one of the plurality of LEDs in the respective one of the plurality of columns and deactivation of the plurality of LEDs in the respective one of the plurality of columns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of an LED system.

FIG. 2 illustrates an example of an LED driver system.

FIG. 3 illustrates an example of a timing diagram.

FIG. 4 illustrates another example of an LED system.

FIG. 5 illustrates another example of a timing diagram.

FIG. 6 illustrates an example of a display system.

FIG. 7 illustrates an example of a method for providing substantially uniform illumination in a light-emitting diode (LED) system.

DETAILED DESCRIPTION

This disclosure relates generally to electronic circuit systems, and more specifically to an LED system with driver voltage clamping. An LED system includes an LED array and an LED controller. As an example, the LED array can include red LEDs, green LEDs, and blue LEDs, and can be implemented in an LED display system (e.g., a television, a computer monitor, or an LED wall). The LED array can be arranged in a plurality of rows and columns. The LED controller can receive a digital input that can include grayscale data corresponding to activation of the LEDs. As an example, the digital input can be provided from an associated image processor. The LED controller can include an activation controller and LED driver systems. The activation controller that can be configured to sequentially activate each of the plurality of rows and can generate activation signals. The activation signals can be provided to the LED driver systems to provide an LED current through the LED in a respective activated row of the LED array to activate the respective LED in an activated state, and to deactivate the LED in a deactivated state.

The sequential activation of the plurality of rows of LEDs can be such that the rows are activated during a first time duration, and all of the rows can be deactivated during a second time duration. As an example, the second time duration can be much longer than a time between activation of consecutive rows during the first time duration. As a result, leakage current from the LEDs during the respective deactivated state can charge respective parasitic capacitors. A driver voltage at an intermediate node that interconnects the LED driver system (e.g., an activation switch of the LED driver system) and the LEDs in the respective column can thus increase slowly in response to the charging of the parasitic capacitor(s) via the leakage current(s), resulting in a greater difference between the driver voltage in the deactivated state and the driver voltage in the activated state. Accordingly, after the second time duration, and thus a relatively long delay prior to activation of the first LED in the column, the LED current flow pulls the driver voltage down to the amplitude in the activated state from a greater amplitude, resulting in a shorter activation time of the LED (e.g., the first activated LED in the column), and thus a non-uniformity of the associated LED display, particularly in low grayscale conditions. Accordingly, to substantially mitigate such non-uniformity, the LED driver system can include a voltage clamp to set a substantially constant amplitude difference between the driver voltage in the deactivated state and the driver voltage in the activated state. As a result, the driver voltage in the deactivated state can be approximately equal for all of the LEDs in a given column, thus resulting in an approximately equal activation time, and thus illumination uniformity, for all of the LEDs in the column.

FIG. 1 illustrates an example of a light-emitting diode (LED) system 10. The LED system 10 can be implemented in a variety of illumination applications, such as in a display system (e.g., a computer monitor, television, or LED wall). The LED system 10 includes an LED array 12 that includes a plurality of LEDs powered by an LED voltage V_(LED). As an example, the LED array 12 can include red LEDs, green LEDs, and blue LEDs that are arranged in an array of rows and columns to provide the respective illumination for a display screen. In the example of FIG. 1, the LED array 12 receives a set of row signals L_A that can sequentially activate the rows of the LEDs in the LED array 12. The LED system 10 also includes an LED controller 14 that is configured to activate the LEDs in the LED array 12 in response to a digital input signal DIG_IN. As an example, the digital input signal DIG_IN can be provided from an image processor (not shown) that is configured to process image data to control activation of sets of the LEDs in the LED array 12 to display an associated image.

In the example of FIG. 1, the LED controller 14 includes an activation controller 16 and LED driver systems 18. The activation controller 16 is configured to generate a set of activation signals based on the digital input DIG_IN, and the LED driver systems 18 are configured to activate the LEDs in the LED array 12. For example, the activation controller 16 can be configured to generate the set of row signals L_A that sequentially activate the rows in the LED array 12. As an example, the LED driver systems 18 can be arranged as any of a variety of amplifier types that are switchably controlled to initiate a current flow through the LEDs in the LED array 12, demonstrated in the example of FIG. 1 as a plurality of currents I_(D1) through I_(DY), where Y is a positive integer. For example, each of the currents I_(D1) through I_(DY) can correspond to a separate respective channel associated with a column of LEDs in the LED array 12, such as scanned individually by row via the row signals L_A. Therefore, the LEDs in the LED array 12 can provide respective portions of illumination based on the currents I_(D1) through I_(DY).

In the example of FIG. 1, each of the LED driver systems 18 includes a voltage clamp 20 that is configured to maintain a driver voltage associated with the currents I_(D1) through I_(DY) at a substantially constant amplitude difference between an activated state (e.g. when the respective one of the currents I_(D1) through I_(DY) is flowing to activate an LED in the LED array 12) and a deactivated state (e.g. when the respective one of the currents I_(D1) through I_(DY) is not flowing, and thus not activating an LED in the LED array 12). As an example, the voltage clamp 20 in each of the LED driver system 18 can be configured as a current source or a resistor that draws current from an intermediate node interconnecting the LED driver systems 18 and the respective column of LEDs and through which the currents I_(D1) through I_(DY) flow. Thus, in response to maintaining the substantially constant amplitude difference of the LEDs between the activated and deactivated states, the voltage clamp 20 can substantially mitigate non-uniformity in the column of LEDs of the LED array 12 based on providing an approximately equal activation time, and thus illumination uniformity, for the LEDs in the respective column of the LED array 12. In the example of FIG. 1, the voltage clamp 20 receives a signal CLMP that can correspond to an input signal for programmably setting the maximum amplitude of the driver voltage, as described in greater detail herein.

FIG. 2 illustrates an example of an LED driver system 50. The LED driver system 50 can correspond to one of the LED driver systems 18 in the example of FIG. 1. Therefore, reference is to be made to the example of FIG. 1 in the following description of the example of FIG. 2.

The LED driver system 50 includes an activation switch N₁, demonstrated in the example of FIG. 2 as an N-channel metal-oxide semiconductor field-effect transistor (MOSFET) having a drain coupled to an intermediate node 52 and a source coupled to a source node 54. As an example, the intermediate node 52 can be coupled to the LEDs in a respective column of the LED array 12 (e.g., at a cathode of each of the LEDs in the column of the LED array 12). The LED driver system 50 also includes an amplifier 56 having an output coupled to the gate of the activation switch N₁ via a switch SW₁, and has a non-inverting input coupled to a reference voltage V_(REF) and an inverting input that is coupled to the source node 54 via a switch SW₂. Additionally, the LED driver system 50 includes a bias transistor N₂, demonstrated as an N-channel MOSFET, interconnecting the source node 54 at a drain and a low-voltage rail (e.g., ground) at a source. The bias transistor N₂ is coupled to a voltage V_(G) at a gate, such as to operate the bias transistor N₂ in linear mode or saturation mode to provide an LED current I_(D) during activation of the activation switch N₁ and the bias transistor N₂.

The switches SW₁ and SW₂ are activated via an activation signal A_ON, such as provided from the activation controller 16 (e.g., as defined by the digital input DIG_IN, such as provided from an image processor). Thus, the activation signal A_ON can be asserted to close the switches SW₁ and SW₂ to couple the output of the amplifier 56 to the gate of the activation switch N₁ and to couple the inverting input of the amplifier 56 to the source node 54. In response, the activation switch N₁ can be activated in linear mode or saturation mode to begin conducting the LED current I_(D) through one of the LEDs in the respective column of the LED array 12 (e.g., in a row that is activated by the row signal L_A). As a result, the LED in the column of the LED array 12 is activated to provide illumination. In the example of FIG. 2, the intermediate node 52 through which the LED current I_(D) flows has a driver voltage V_(DRV) that has a first amplitude in a deactivation state, when the activation switch N₁ is deactivated and the LED current I_(D) is not flowing, and a second amplitude that is less than the first amplitude in an activated state, when the activation switch N₁ is activated and the LED current I_(D) is flowing. Accordingly, the driver voltage V_(DRV) decreases from the first amplitude to the second amplitude in response to activation of the activation switch N₁, and thus the LED current I_(D).

To deactivate the activation switch N₁, the activation signal A_ON is de-asserted and a deactivation signal A_OFF is asserted to close a switch SW₃ interconnecting the gate of the activation switch N₁ and the low-voltage rail. As an example, the deactivation signal A_OFF can be an inverted version of the activation signal A_ON, or can be a separate signal. Therefore, the voltage at the gate of the activation switch N₁ is pulled low to deactivate the activation switch N₁, ceasing the flow of the LED current I_(D). The driver voltage V_(DRV) increases from the second amplitude back to the first amplitude, and the LED driver system 50 is held in the deactivated state until the next LED in the column is to be activated (e.g., in response to the sequential activation of the next consecutive row of LEDs in the LED array 12.

In the deactivated state, the LEDs in the column of the LED array 12 can exhibit a leakage current that can build a charge on a parasitic capacitor of each of the LEDs in the column of the LED array 12. As an example, the leakage current can be very small (e.g., in the scale of nanoamps). As a result, in the deactivated state, the driver voltage V_(DRV) can slowly increase over time, with the increase of the driver voltage V_(DRV) being greater in response to longer durations of the deactivated state. As an example, the activation controller 16 can provide the set of row signals L_A during a first time duration to sequentially activate the rows of the LED array 12 for selective activation of the LEDs in each of the columns based on the activation signals (e.g., A_ON) via the respective LED driver systems (e.g., the LED driver system 50). After the sequential activation of the rows, the activation controller 16 can deactivate all of the rows for a second duration of time, which can be longer than a time between consecutive row activations in the sequence via the row signals L_A. As an example, the second time duration can be longer than the first time duration. As a result, the driver voltage V_(DRV) can increase by a fraction of volt. However, because the rate of decrease of the driver voltage V_(DRV) is substantially constant at the beginning of each activated state, a greater amplitude of the driver voltage V_(DRV), such as greatest for activation of the first LED activated in the sequence of the activated rows, could result in less activation time for a given pulse of the activation signal A_ON. As described herein, the term “activation time” refers to a duration of time during which a given LED is activated and providing illumination. Accordingly, such difference in activation time can result in non-uniformity of an associated LED display, such as to provide a “first line dark” problem, which refers to a darker first line corresponding to a first row of sequentially activated LEDs relative to other rows in the sequence (e.g., in a low grayscale condition).

To substantially mitigate non-uniformity, and thus a potential first line dark problem, the LED driver system 50 includes a voltage clamp 58. The voltage clamp 58 is demonstrated in the example of FIG. 2 as a current source that provides a current I_(CLMP) flowing from the intermediate node 52 to the low-voltage rail. The current I_(CLMP) can be set, for example, based on a signal CLMP, such that the voltage clamp 58 can be programmable. As an example, the signal CLMP can be a signal that can be adjusted in real-time to provide dynamic changes to the current I_(CLMP), or can be a more static setting, such as at fabrication or via dip switches or the like, to set the current I_(CLMP) to a given magnitude. As an example, the current I_(CLMP) can be set approximately equal to the leakage current of the LEDs in the column (e.g., a sum of the leakage current of all of the LEDs in the column). As a result, the current I_(CLMP) can clamp the driver voltage V_(DRV) to a maximum amplitude in the deactivated state, and thus to maintain a constant amplitude difference between the value of the driver voltage V_(DRV) in the activated state and the deactivated state.

FIG. 3 illustrates an example of a timing diagram 100. The timing diagram 100 demonstrates the activation signal A_ON at 102, the driver voltage V_(DRV) as affected by the voltage clamp 58 at 104, and a driver voltage V_(DRV)* corresponding to an absence of the voltage clamp 58 at 106, such as for a first LED in a column of sequentially activated rows. The timing diagram 100 thus demonstrates changes in amplitude of the driver voltage V_(DRV) in the activated and deactivated states. In the example of FIG. 3, the driver voltages V_(DRV) and V_(DRV)* are demonstrated relative to a first amplitude V₁, a second amplitude V₂, and a third amplitude V₃. The first amplitude V₁ (e.g., approximately 2.87 or more volts) can correspond to the driver voltage V_(DRV) in the deactivated state at an increased voltage amplitude resulting from leakage current from the LEDs in the column charging respective parasitic capacitors over a duration of time (e.g., the second duration of time corresponding to deactivation of all of the rows of the LEDs in the LED array 12, as described previously). The second amplitude V₂ (e.g., approximately 2.63 volts) can correspond to a voltage amplitude of the deactivated state, without additional amplitude resulting from leakage current charging respective parasitic capacitors. The third amplitude V₃ (e.g., approximately 2.0 volts) can correspond to the driver voltage V_(DRV) in the activated state, such that the LED current I_(D) flows through an LED in the column to provide illumination. While the example of FIG. 3 does not include the deactivation signal A_OFF, it is to be understood that the deactivation signal A_OFF can be an inverted version of the activation signal A_ON, and thus can have an opposite logic state as the activation signal A_ON at any given time in the timing diagram 100.

Prior to a time T₀, the activation signal A_ON is de-asserted. Therefore, the LED driver system 50 is in the deactivated state, such that the activation switch N₁ is deactivated based on the switches SW₁ and SW₂ being open and the switch SW₃ being closed. At 104, it is demonstrated that the driver voltage V_(DRV) has the amplitude V₂, which can correspond to a maximum voltage of the driver voltage V_(DRV) as set by the voltage clamp 58. For example, the voltage clamp 58 can draw a current from the intermediate node 52 at a magnitude that is approximately equal to a sum of leakage currents of the LEDs in the column of the LED array 12. Therefore, the driver voltage V_(DRV) can be maintained at the voltage amplitude V₂ as a maximum amplitude at all times between activations of the activation switch N₁ (i.e., during the first duration and the second duration of time). However, at 106, it is demonstrated that the driver voltage V_(DRV)* has the amplitude V₁, greater than the amplitude V₂, which can result from the absence of the voltage clamp 58, and thus based on a steady slow increase of the driver voltage V_(DRV) in response to leakage current of the LEDs charging the associated parasitic capacitors.

At a time T₀, the activation signal A_ON is asserted, such that the switch SW₃ is opened and the switches SW₁ and SW₂ are closed to activate the activation switch N₁. As a result, the LED current I_(D) begins to flow to activate an LED (e.g., the first LED in a column that is sequentially activated by row via the row signals L_A). In response to the flow of the LED current I_(D), the driver voltage V_(DRV) begins to decrease. At 104, it is demonstrated that the driver voltage V_(DRV) begins to decrease from the amplitude V₂ based on the flow of the LED current I_(D). The driver voltage V_(DRV) decreases down to the amplitude V₃ at a time T₁, and thus remains at the amplitude V₃ during the activated state. Also at the time T₀, the driver voltage V_(DRV)* begins to decrease. However, as demonstrated at 106, the driver voltage V_(DRV)* begins to decrease from the amplitude V₁, greater than the amplitude V₂, based on the flow of the LED current I_(D). Because the rate of decrease of the driver voltage V_(DRV) does not depend on the presence of the voltage clamp 58, the rate of decrease of the driver voltage V_(DRV)* is the same as the decrease of the driver voltage V_(DRV). Thus, the driver voltage V_(DRV)* likewise decreases down to the amplitude V₃, but because the amplitude V₁ is greater than the amplitude V₂, the driver voltage V_(DRV)* decreases down to the amplitude V₃ at a time T₂, which is subsequent to the time T₁.

At a time T₃, the activation signal A_ON is de-asserted, such that the switches SW₁ and SW₂ are opened and the switch SW₃ is closed to deactivate the activation switch N₁. As a result, the LED current I_(D) no longer flows, thus deactivating the associated LED. In response to the cessation of the LED current I_(D), the driver voltage V_(DRV) and the driver voltage V_(DRV)* begin to increase, reaching the amplitude V₂ at a time T₄. At a time subsequent to T₄, at 104, the driver voltage V_(DRV) is maintained at a substantially constant amplitude at the amplitude V₂ based on the voltage clamp 58 drawing the current I_(D) from the intermediate node 52. However, at 106, without the voltage clamp 58, the driver voltage V_(DRV)* begins to slowly increase based on the leakage current from the LEDs charging the parasitic capacitors.

As demonstrated at 104 and 106, the driver voltage V_(DRV) is maintained at the amplitude V₃ for a longer duration of time than the driver voltage V_(DRV)*. In the example of FIG. 3, the driver voltage V_(DRV) is held at the amplitude V₃, and thus in the activated state to provide illumination from the respective LED, from the time T₁ to the time T₃. Conversely, the driver voltage V_(DRV)* is held at the amplitude V₃, and thus in the activated state to provide illumination from the respective LED, from the amount of time T₂ to the time T₃, and thus a shorter duration than the driver voltage V_(DRV). Accordingly, the amount of illumination of the respective LED in the column (e.g., the first LED in the sequentially activated rows) provides less illumination in the example at 106 relative to the example at 104. In the example of the LED being the first LED in the sequentially activated rows, and thus after the second duration of time that is longer than a time between sequential activations of the consecutive rows, the driver voltage V_(DRV)* can have the highest amplitude prior to the activated state relative to the activated state of the other LEDs in the column, thus creating the first line dark problem. Accordingly, the voltage clamp 58 can provide the maximum amplitude of the driver voltage V_(DRV) in the deactivated state to maintain a substantially constant amplitude difference between the activated state and the deactivated state for substantially all of the LEDs, thus providing substantial illumination uniformity across an associated LED display.

FIG. 4 illustrates another example of an LED system 150. The LED system 150 can correspond to the LED system 10 in the example of FIG. 1. Therefore, reference is to be made to the example of FIG. 1 in the following description of the example of FIG. 2.

The LED system 150 includes an LED array 152 that includes a plurality X of rows of LEDs and a plurality Y of columns of LEDs, where X and Y are each positive integers. Thus, the first row of LEDs are designated as D_(1_1) to D_(1_Y) and a last row of LEDs are designated as D_(X_1) to D_(X_Y), corresponding to a position in the LED array 152 designated by row_column. As an example, the LEDs can be organized by color in a given row and/or column (e.g., red, green, and/or blue). In the example of FIG. 4, the LEDs in the LED array 152 receive power from an LED voltage V_(LED) via respective row switches SW_(R1) to SW_(RX) that are activated by respective row signals L1_A to LX_A, such as generated by the activation controller 16. Thus, the activation controller 16 can sequentially activate the rows of LEDs in the LED array 152 via the respective switches SW_(R1) through SW_(RX) to couple the respective rows to the voltage V_(LED). Similarly, the activation controller 16 can deactivate a given row by opening the respective one of the switches SW_(R1) through SW_(RX) and by closing one of a respective set of switches SW_(D1) through SW_(DX) to couple the respective row to a low-voltage rail (e.g., ground) via a respective resistor R1 through RX. As an example, during the first time duration, one of the rows of the LED array 152 can be activated at a time in a sequence (e.g., row 1 to row X), and during the second duration of time, all of the rows of the LED array 152 can be deactivated.

The LED system 150 also includes an LED controller 154 that includes a plurality of LED driver systems 156 that are each associated with a respective one of the columns 1 through Y of the LED array 152. The LED driver systems 156 are each activated in response to respective activation signals A1 through AY, that can each include the activation signal A_ON and the deactivation signal A_OFF. Thus, the activation signals A1 through AY can include the activation signal A_ON as described in the example of FIGS. 2 and 3 to provide a respective current flow I_(D1) through I_(DY) to activate the respective LED in the column that corresponds to the activated row. Similarly, the activation signals A1 through AY can include the deactivation signal A_OFF as described in the example of FIG. 2 to cease the respective current flow I_(D1) through I_(DY) to deactivate the LEDs in the column.

In the example of FIG. 4, each of the LEDs D_(1_1) through D_(X_Y) includes a respective parasitic capacitor C_(P1_1) through C_(PX_Y) that is arranged in parallel with the respective LED. In a deactivated state, the LEDs D_(1_1) through D_(X_Y) can provide a leakage current that can charge the respective parasitic capacitors C_(P1_1) through C_(PX_Y), thus increasing respective driver voltages V_(DRV1) through V_(DRVY) at intermediate nodes 158 that interconnect the LEDs D_(1_1) through D_(X_Y) with the respective LED driver systems 156. Therefore, to substantially mitigate the increase of the respective driver voltages V_(DRV1) through V_(DRVY) during a deactivated state of the LEDs D_(1_1) through D_(X_Y), and thus to substantially mitigate non-uniform illumination of the LEDs D_(1_1) through D_(X_Y), the LED driver systems 156 each include a voltage clamp 160. As an example, the voltage clamps 160 can be configured to conduct a current I_(CLMP) from the respective intermediate nodes 158, thus setting a substantially constant amplitude difference between the driver voltage V_(DRV1) through V_(DRVY) in each of the activated and deactivated states, such as described previously regarding the example of FIGS. 2 and 3. For example, the voltage clamps 160 can be configured as current sources or as resistors, which can be static or programmable, to conduct the current I_(CLMP) that can have a magnitude, for example, that is approximately equal to the leakage current of the LEDs D_(1_1) through D_(X_Y).

FIG. 5 illustrates another example of a timing diagram 200. The timing diagram 200 can correspond to a timing diagram for the LED system 150 in the example of FIG. 4. Therefore, reference is to be made to the example of FIG. 4 in the following description of the example of FIG. 5. The timing diagram 200 can correspond to a specific column of LEDs of the LED array 152.

The timing diagram 200 demonstrates the clock signal CLK, the row signals L1_A through LX_A, the activation signal A_ON, the driver voltage V_(DRV) of a respective one of the LED driver systems 156, and the driver voltage V_(DRV)* corresponding to a respective hypothetical LED driver that does not include a voltage clamp 160, similar to as described previously in the example of FIG. 5. At a time T₀, the first time duration, demonstrated at 202, begins. Thus, at the time T₀, the activation controller 16 begins to sequentially activate the rows of LEDs of the LED array 152. At the time T₀, the row signal L1_A is asserted to activate the first row (e.g., row 1 via the switch SW_(R1)). The activation signal A_ON of the respective LED driver system 156 is thus concurrently asserted with the row signal L1_A. Accordingly, the respective current I_(D) flows through the activated LED (e.g., LED D1_1) to provide illumination. The driver voltage V_(DRV) thus decreases from a deactivated state amplitude (e.g., the amplitude V₂) to an activated state amplitude (e.g., the amplitude V₃). Additionally, driver voltage V_(DRV)* decreases from a deactivated state amplitude (e.g., the amplitude V₁) that is greater than the deactivated state amplitude of the driver voltage V_(DRV) (e.g., the amplitude V₂) to the activated state amplitude (e.g., the amplitude V₃). As an example, the deactivated state amplitude of the driver voltage V_(DRV)* can be based on the charging of parasitic capacitors (e.g., C_(P1_1) through C_(PX_1)) of the LEDs via respective leakage currents, such as during a previously occurring second time duration.

At a time T₁, the row signal L1_A is de-asserted to deactivate the first row (e.g., row 1 via the switch SW_(D1)). The activation signal A_ON of the respective LED driver system 156 is thus concurrently de-asserted with the row signal L1_A. As an example, at the time T₁, the row signal L1_D and the deactivation signal A_OFF (not shown, respectively) can be asserted. Accordingly, the respective current I_(D) ceases to flow through the respective LED (e.g., LED D_(1_1)), thus deactivating the LED. In the example of FIG. 5, the row signal L1_A and the activation signal A_ON thus have a duration of one period of the clock signal CLK. However, it is to be understood that the relationship between the row signals L_A and the activation signal A_ON relative to the clock signal CLK can be different than as demonstrated in the example of FIG. 5. At a time T₂, the row signal L2_A is asserted to activate the second row (e.g., row 2 via the switch SWR2). The activation signal A_ON of the respective LED driver system 156 is thus concurrently asserted with the row signal L2_A. Accordingly, the respective current I_(D) flows through the activated LED (e.g., LED D1_2) to provide illumination. Both the driver voltage V_(DRV) and the driver voltage V_(DRV)* decrease from approximately the same amplitude. For example, the charging of the parasitic capacitors via the leakage current may be substantially negligible in the short time between the times T₁ and T₂.

Therefore, the first time duration continues with sequential activation of the rows 1 through X via the row signals L1_A through LX_A, as described previously. At a time T₃, the last row X of the LEDs in the LED array 152 is deactivated, thus concluding the first time duration 202 (e.g., from the time T₀ to the time T₃) and beginning the second time duration 204. During the second time duration 204, none of the rows 1 through X are activated via the row signals L1_A through LX_A, and the activation signal A_ON is not asserted. As an example, the second time duration 204 can be much greater than a time between sequential activation of consecutive rows (e.g., between the time T₁ and T₂). For example, the second time duration 204 can be greater than the first time duration 202 (e.g., having more cycles of the clock signal CLK). During the second time duration 204, the voltage clamp 160 in the LED driver system 156 can provide the current flow I_(CLMP) from the intermediate node 158 to set a maximum amplitude of the driver voltage V_(DRV) in the deactivated state (e.g., at the amplitude V₂). Therefore, the driver voltage V_(DRV) is held at a substantially constant amplitude during the second time duration 204. However, absent the voltage clamp 160, the driver voltage V_(DRV)* is demonstrated as slowly increasing during the second time duration 204 to an amplitude (e.g., the amplitude V₁) that is greater than a deactivated state magnitude between activations of consecutive rows (e.g., between the time T₁ and the time T₂). It is to be understood that the increased amplitude of the driver voltage V_(DRV)* is exaggerated in the example of FIG. 5 by scale for the sake of demonstration.

At a time T₄, another first time duration, demonstrated at 206, begins. Thus, at the time T₄, the activation controller 16 again begins to sequentially activate the rows of LEDs of the LED array 152. At the time T₀, the row signal L1_A is asserted to again activate the first row (e.g., row 1 via the switch SW_(R1)). The activation signal A_ON of the respective LED driver system 156 is thus concurrently asserted with the row signal L1_A. Accordingly, the respective current I_(D) flows through the activated LED (e.g., LED D_(1_1)) to provide illumination. The driver voltage V_(DRV) thus decreases from the maximum deactivated state amplitude (e.g., the amplitude V₂) set by the voltage clamp 160 to the activated state amplitude (e.g., the amplitude V₃). However, the driver voltage V_(DRV)* decreases from the increased deactivated state amplitude (e.g., the amplitude V₁) that is greater than the deactivated state amplitude of the driver voltage V_(DRV) (e.g., the amplitude V₂) to the activated state amplitude (e.g., the amplitude V₃). Therefore, the amount of time that the LED in the first row (LED D_(1_1)) is at the activated state voltage, and thus is activated to provide illumination, is less than the amount of time that the LEDs in the other rows are at the activated state voltage, and thus is activated to provide illumination. Accordingly, the LED display experiences the first line dark problem, resulting in a non-uniformity of the LED display. As a result, by incorporating the voltage clamp 160 in each of the LED driver systems 156, the difference of the driver voltage V_(DRV) between the activated state and the deactivated state is substantially constant for each of the LEDs, thus providing an approximately equal amount of activation time for each of the LEDs in a given column. Accordingly, the LED system 150 can provide substantially uniform illumination across the entirety of the respective portion of an LED display surface.

FIG. 6 illustrates an example of a display system 250. The display system 250 can correspond to a display system for a computer, such as for a computer monitor, an LED television, or an LED wall. The display system 250 includes an image processor 252 that can be configured to generate image data, such as in response to a broadcast communication signal in a television system or from a processor in a computer system. The display system 250 also includes an LED display 254 that receives the image data, demonstrated as the digital input DIG_IN in the example of FIG. 6. As an example, the LED display 254 can be an LED television, a computer monitor, or an LED wall. The LED display 254 includes an LED array 256, such as similar to the LED array 152 in the example of FIG. 4, and an LED controller 258, such as similar to the LED controller 14 in the example of FIG. 1 or the LED controller 154 in the example of FIG. 4. As a result, the LED controller 258 can include LED driver systems, such as similar to the LED driver system 50 in the example of FIG. 2. As an example, each of the LED driver systems can include a voltage clamp configured to conduct a current from an intermediate node that interconnects the respective LED driver system from the column of LEDs in the LED array 256. Accordingly, the LED display 254 can provide illumination in a substantially uniform manner based on the voltage clamps setting a maximum driver voltage at the intermediate node in a deactivated state, as described herein.

In view of the foregoing structural and functional features described above, certain methods will be better appreciated with reference to FIG. 7. It is to be understood and appreciated that the illustrated actions, in other embodiments, may occur in different orders and/or concurrently with other actions. Moreover, not all illustrated features may be required to implement a method.

FIG. 7 illustrates an example of a method 300 for providing substantially uniform illumination in an LED system (e.g., the LED system 10). At 302, a plurality of rows of LEDs (e.g., the rows 1 through X) arranged in an array (e.g., the LED array 12) are sequentially activated. At 304, a switch (e.g., the activation switch N₁) is activated in response to an activation signal (e.g., the activation signal A_ON) to conduct an LED current (e.g., the current I_(D)) through an LED (e.g., the LED D_(1_1)) in a respective activated one of the plurality of rows of LEDs to activate the LED. The switch can be coupled to the LED via an intermediate node (e.g., the intermediate node 52) having a driver voltage (e.g., the driver voltage V_(DRV)). At 306, the switch is deactivated via a deactivation signal (e.g., the deactivation signal A_OFF) to deactivate the LED. At 308, a clamping current (e.g., the clamping current I_(CLMP)) is conducted from the intermediate node to set a maximum amplitude of the driver voltage while the switch is deactivated.

What have been described above are examples of the invention. It is, of course, not possible to describe every conceivable combination of components or method for purposes of describing the invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the invention are possible. Accordingly, the invention is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims. 

What is claimed is:
 1. A circuit comprising: an amplifier having an amplifier input and an amplifier output; a first transistor having first and second terminals and a first gate; a second transistor having third and fourth terminals and a second gate, wherein the first and third terminals are coupled together; a first switch coupled between the amplifier output and the first gate of the first transistor; a second switch having a first end coupled to the amplifier input and a second end coupled to the first and third terminals; and a voltage clamp coupled to the second terminal of the first transistor, wherein the voltage clamp is configured to conduct current to restrict a voltage at the second terminal during a state in which the first and second switches are open.
 2. The circuit of claim 1, wherein the voltage clamp includes a resistor.
 3. The circuit of claim 2, wherein the resistor is an adjustable resistor.
 4. The circuit of claim 1, wherein the voltage clamp includes a current source.
 5. The circuit of claim 4, wherein the current source is a programmable current source configured to receive an input signal and responsively conduct the current.
 6. The circuit of claim 1, wherein the current is based on a leakage current of a light-emitting diode.
 7. The circuit of claim 1, wherein the amplifier input is an inverting input, and the transistors are N-channel metal-oxide semiconductor field-effect transistors.
 8. The circuit of claim 1, further comprising light-emitting diodes (LEDs) coupled to the second terminal of the first transistor.
 9. The circuit of claim 8, wherein the current is based on a leakage current of the LEDs during the state in which the first and second switches are open.
 10. The circuit of claim 1, further comprising a third switch coupled between the first gate of the first transistor and a ground reference, wherein: the fourth terminal of the second transistor is coupled to the ground reference; the voltage clamp is coupled between the second terminal of the first transistor and the ground reference; the state is a first state, further in which the third switch is closed; and the transistors are configured to conduct current during a second state in which the first and second switches are closed and the third switch is open.
 11. The circuit of claim 1, wherein the state is a first state, and the voltage clamp is configured to maintain a substantially constant voltage differential between first and second voltages at the second terminal of the first transistor, the first voltage is responsive to the current conducted by the voltage clamp, and the second voltage is responsive to the transistors conducting current during a second state in which the first and second switches are closed.
 12. A system comprising: LEDs; and a controller coupled to the LEDs, the controller including a driver circuit including: an amplifier having an amplifier input and an amplifier output; a first transistor having first and second terminals and a first gate; a second transistor having third and fourth terminals and a second gate, wherein the first and third terminals are coupled together; a first switch coupled between the amplifier output and the first gate of the first transistor; a second switch having a first end coupled to the amplifier input and a second end coupled to the first and third terminals; and a voltage clamp coupled to the second terminal of the first transistor, wherein the voltage clamp is configured to conduct current to restrict a voltage at the second terminal during a state in which the first and second switches are open.
 13. The system of claim 12, wherein the LEDs and the controller are included in a display of the system, and the system further comprising an image processor coupled to the display.
 14. The method of claim 12, wherein the driver circuit is a first driver circuit, the amplifier is a first amplifier having a first amplifier input and a first amplifier output, and the voltage clamp is a first voltage clamp, and the controller further includes a second driver circuit including: a second amplifier having a second amplifier input and a second amplifier output; a third transistor having fifth and sixth terminals and a third gate; a fourth transistor having seventh and eighth terminals a fourth gate, wherein the fifth and seventh terminals are coupled together; a third switch coupled between the second amplifier output and the third gate of the third transistor; a fourth switch having a respective first end coupled to the second amplifier input and a respective second end coupled to the fifth and seventh terminals; and a second voltage clamp coupled to the sixth terminal of the third transistor, wherein the second voltage clamp is configured to conduct current to restrict a voltage at the sixth terminal during a state in which the third and fourth switches are open.
 15. The system of claim 14, wherein the LEDs are arranged into columns including a first column of LEDs coupled to the first driver circuit and a second column of LEDs coupled to the second driver circuit.
 16. The system of claim 12, wherein the voltage clamp includes a resistor.
 17. The system of claim 12, wherein the voltage clamp includes a current source.
 18. The system of claim 12, wherein the current is based on a leakage current of one or more of the LEDs.
 19. The system of claim 12, wherein the amplifier input is an inverting input, and the transistors are N-channel metal-oxide semiconductor field-effect transistors.
 20. A circuit comprising: an amplifier having an inverting input and an amplifier output; a first transistor having a source, a drain, and a gate; a second transistor having a respective source, a respective drain, and a respective gate, wherein the source of the first transistor is coupled to the drain of the second transistor; a first switch coupled between the amplifier output and the gate of the first transistor; a second switch having one terminal coupled to the inverting input and another terminal coupled to the source of the first transistor and the drain of the second transistor; and a voltage clamp coupled to the drain of the first transistor, wherein the voltage clamp is configured to conduct current to restrict a voltage at the drain of the first transistor during a state in which the first and second switches are open.
 21. The circuit of claim 20, wherein the voltage clamp includes a resistor.
 22. The circuit of claim 20, wherein the voltage clamp includes a current source.
 23. The circuit of claim 20, wherein the current is based on a leakage current of a light-emitting diode.
 24. The circuit of claim 20, further comprising light-emitting diodes (LEDs) coupled to the drain of the first transistor.
 25. A method comprising: closing switches during a first time period to couple an amplifier output to a gate of a first transistor and to couple an amplifier input to a first terminal of the first transistor and a respective first terminal of a second transistor; conducing current using the transistors during the first time period; opening the switches during a second time period; and conducting current during the second time period using a voltage clamp to clamp a voltage at a second terminal of the first transistor.
 26. The method of claim 25, wherein conducing the current using the voltage clamp includes conducting the current using a resistor.
 27. The method of claim 25, wherein conducing the current using the voltage clamp includes conducting the current using a current source.
 28. The method of claim 25, wherein the switches include first and second switches, the method further comprising: closing a third switch during the second time period to couple the gate of the first transistor to a ground reference; and opening the third switch during the first time period.
 29. The method of claim 25, further comprising illuminating a light-emitting diode using the current conducted during the first time period. 